Thanks Lynn,
I think the fact that you included the
term “effective AOA” makes it easier for me to grasp. The prop just becomes
better at what it does the faster you go, and it isn’t designed to operate at
no or low airspeed. I appreciate your time and patience.
BW
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of Lynn Hanover
Sent: Thursday, March 04, 2010
7:40 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] The stalled
propeller blade
Veeeerrrrrry interesting indeed. I think the key difference
is, not so much the greater HP, but the greater torque (albeit via HP) to turn
that larger prop.
One thing I want to avoid is that accident you referred to with too
fine pitch. For now I have an electric MT with auto pitch control.
That should help me avoid problems like this.
I have a lot of jet time, but little prop time. So when it
comes to AOA, I think in terms of the wing producing max lift. The AOA
changes with elevator input. A prop is not unlike a wing, but I still
don’t have a grasp of what is causing AOA change in a fixed pitch prop.
But, usually I have to read things three or four times in order to “get it”.
What gave rise to seeing a need for clipping the larger prop?
Was the torque not there? I’m sure you answered this a few days ago, I’ll
look over the old posts.
And yeah, it’s possible to overthink this thing.
Fun nonetheless.
The propeller blade is a wing, but used in
an impossible situation.
To stall or not to stall that is the
question.
A stalled airfoil has some of the smooth
flow near the trailing edge separating from the surface and rolling.
This reduces the mass of air that is
changing direction (Vector) and the thrust of the blade or the lift of the wing
is reduced.
This is a function of Angle of attack to
the RELATIVE wind or airflow. When the plane is tied to a tree with a spring
scale, or just sitting still, the flow through the propeller disc is nearly at
90 degrees to the zero lift line of the prop. This flow changes for the worst
as the propeller RPM goes up. The cylinder of air going through the prop begins
to accelerate reducing the (Effective) angle of attack of the blades, however
the last bit of the airflow as it enters the disc begins to spin or, follow the
blades around a bit. This increases the Angle of attack.
The actual blade angle relative to the
crankshaft never changes. The blades path through an ever faster moving column
of air produces a lengthening spiral through the column. If you look at the
angular difference between the spiral path and the zero lift line of the blade,
you see the effective angle of attack.
As the aircraft and, or the air column
accelerates the distance between the points on this spiral will be further
apart and the effective angle of attack will be further reduced. So the fixed
pitch prop changes angle of attack to the relative wind as a function of the
aircrafts foward velocity. So your RV whatever may not take full throttle from
stopped. But once it is moving 15 or 20 MPH can take all you have.
Because the effective angle of attack of
the blades has been reduced, unstalling part of the prop.
In the stationary aircraft:
Also the cylinder of air is moving into
the prop because the prop has generated a low pressure area just in front of
the disc.
So local air outside of this moving column
is at a higher pressure than the air in the column, and begins to compress the
column right in front of the disc.
Now the prop tips may not be seeing a
column of air moving as fast as the mid blade sees, and the tips may be stalled
and nothing else is.
The air leaving the back side of the
blades is accelerated violently and Bernoulli using more local unmoving air
chokes the fleeing cylinder of fast moving low pressure air tight against
the fuselage. The cylinder of air is now spinning (P factor) and its low
pressure is causing slow moving local air to mix with it, so it is gaining
mass, And raising the speed of the slower air.
So if the prop is small in diameter and
turning very fast it may produce a higher velocity column, but of reduced mass.
Conversely, a large diameter prop when the same torque is applied will turn
more slowly, but move more mass per second, and attract more local air and
accelerate that as well. Disc area is the easy way to express all of
this.
More is better. More HP is better. More
torque is better. More diameter (Disc area) is better. More blades are better.
Wider blades are better. Lighter blades are better. But not for everything.
This should get us down to about 1,000
additional factors to think about.
Static thrust. Not just a good idea.
Ask the guy in the black biplane hovering
200 feet above the concrete at Sun&Fun. That's over 1,000 pounds of static
thrust.